In the process of cutting quartz,machining errors often cause two surfaces unparallel,which will can influence on the judgment to its crystal orientation.The angle measurement of fullautomated xray sorting machine was analyzed.The light spots deflected by quartz were detected by position sensitive detector,and relational expression between points position sensitive detector detected in the coordination and angles between top and bottom surfaces was deduced.And,the shapes of the light spots on the position sensitive detector in different incident angles and thicknesses were studied.It was found that similar linear relationship exists between height differences at coordination axis on position sensitive detector.When the incident angle is 20°，the norm of residual has the smallest value of 5.306 2×10-6 mm.The system resolution and measurement range are 1.033″ and 15′ respectively.